A thin film transistor (hereinafter sometimes abbreviated as “TFT”) is widely used as a switching device for display of a liquid crystal display, etc. A sectional structure of a representative TFT is shown in FIG. 1. As shown in FIG. 1, the TFT has a gate electrode, an insulator layer and an organic semiconductor layer in this order on a substrate and has a source electrode and a drain electrode formed at a prescribed interval on the organic semiconductor layer. A part of each of the source electrode and the drain electrode is exposed on the surface, and the semiconductor layer is formed on the surface exposing between the both electrodes. In the TFT having such a configuration, the semiconductor layer forms a channel region, a current flowing between the source electrode and the drain electrode is controlled by a voltage to be applied to the gate electrode, and the TFT undergoes an ON/OFF operation.
A TFT has hitherto been prepared using amorphous or polycrystalline silicon. However, a CVD apparatus which is used for the preparation of a TFT using such silicon is very expensive, and therefore, there was involved such a problem that increasing in size of a display, etc. using a TFT is accompanied by a big increase of manufacturing costs. Also, since a process for depositing amorphous or polycrystalline silicon is carried out at a very high temperature, the kind of a material which can be used as a substrate is limited. Thus, there was encountered such a problem that a lightweight polymer film substrate, to which flexibility can be imparted and which is able to be freely subjected to shape design, or the like cannot be used. If it is possible to manufacture a TFT on a lightweight polymer film substrate, it is expected that an application to portable electronic devices will become possible.
In order to solve such problems, there has been proposed a TFT using an organic semiconductor (hereinafter sometimes abbreviated as “organic TFT”). As a deposition method which is adopted during the formation of a TFT by an organic semiconductor, there are known a vacuum vapor deposition method, a coating method and so on. According to such a deposition method, it is possible to realize to increase the size of a device while suppressing an increase of the manufacturing costs, and the process temperature which is necessary at the time of deposition can be made relatively low. Also, in the TFT using an organic material, there is an advantage that limitations at the time of selection of a material to be used for the substrate are few, and its practical implementation is expected. Under such a circumstance, there have been made a number of research reports regarding the organic TFT. For example, Non-Patent Documents 1 to 3 can be enumerated. Also, as a material to be used for a semiconductor layer of TFT, so far as a p-type is concerned, multimers such as conjugated polymers, and thiophenes (Patent Document 1); condensed aromatic hydrocarbons such as pentacene (Patent Document 2); and the like are known. Also, as a material of an n-type FET, 1,4,5,8-naphthalenetetracarboxyl dianhydride (NTCDA), 11,11,12,12-tetracyanonaphth-2,6-quinodimethane (TCNNQD), 1,4,5,8-naphthalenetetracarboxyl diimide (NTCDI) and the like are disclosed in Patent Document 3.
Similar to the organic TFT, there is known an organic electroluminescence device (hereinafter sometimes abbreviated as “organic EL device”) as a device using electric conduction. The organic EL device generally forcedly feeds charges upon applying a strong electric field of 105 V/cm or more in a thickness direction to a ultra-thin film of not more than 100 nm; whereas in the case of the organic TFT, it is necessary to feed charges at a high speed over a distance of several μm or more in an electric field of not more than 105 V/cm. For that reason, the organic material itself is required to have conductivity exceeding the organic EL device material. However, organic materials which are used in the conventional organic TFTs are low in a field-effect mobility (hereinafter sometimes abbreviated as “mobility”) and slow in a response speed. For that reason, there was involved a problem in high-speed response as a transistor. Also, an ON/OFF ratio was small.
The “ON/OFF ratio” as referred to herein means a value obtained by dividing a current (ON current) flowing between a source and a drain when a gate voltage is applied, namely in an ON state by a current flowing between the source and the drain when a gate voltage is not applied, namely in an OFF state. Also, the “ON current” as referred to herein means a current value (saturated current) when the current flowing between the source and the drain is saturated at the time of increasing the gate voltage.
There have been made attempts to solve these problems of organic TFTs and to enhance the mobility by an improvement of the device configuration. For example, it is attempted to insert a vapor deposited film of N,N-dinaphthalen-1-yl-N,N′-diphenyl-biphenyl-4,4′-diamine (NPD) at an interface between a channel layer (organic semiconductor layer) and an insulating layer (Non-Patent Document 4). However, since NPD is low in glass transition temperature and insufficient in heat resistance, there was involved such a problem that in the case where it is put into practical use at a high temperature, its mobility is lowered with time. Also, on the contrary, in order to obtain an organic TFT which is stable against heat, it is attempted to use a polymer such as polystyrene upon being crosslinked (Non-Patent 5). However, there was involved such a problem that when only this is applied, a high mobility is not obtained.    [Patent Document 1] JP-A-8-228034    [Patent Document 2] JP-A-5-55568    [Patent Document 3] JP-A-10-135481    [Non-Patent Document 1] Horowitz, et al., Advanced Materials, Vol. 8, No. 3, page 242 (1996)    [Non-Patent Document 2] H. Fuchigami, et al., Applied Physics Letter, Vol. 63, page 1372 (1993)    [Non-Patent Document 3] Lay-Lay Chua, et al., Nature, Vol. 434, Mar. 10, 2005, page 194    [Non-Patent Document 4] Ishikawa, et al., Preprint of The 54th Spring Meeting, JSAP and Related Societies, 2007, 30a-W-11, page 1424    [Non-Patent Document 5] Myung-Han Yoon, et al., J. AM. CHEM. SOC, 2005, 127, 10388 to 10395